1. Field of the Invention
This invention relates to an image coding apparatus for quantizing and coding an input image signal, and more particularly to a method of determining an optimum quantization step in an image coding apparatus.
2. Description of the Related Art
Various image coding apparatus for coding an input image signal which employ various methods are known, and one of such image coding apparatus is an image coding apparatus which makes use of subband division.
In an image coding apparatus which makes use of subband division, an input image signal is divided into a plurality of subband signals having different frequency bands from each other, and a suitable bit is distributed to each of the subbands and each of the subband signals is represented with a number of bits smaller than the number of bits of the original signal to obtain a high coding efficiency.
FIG. 7 shows the construction for performing subband division. In subband division, an input image signal S0 is divided into a plurality of signals of different frequency bands, that is, subband signals S1 and S2, by a pair of digital filters F1 and F2, respectively. Such subband division is performed for signals in a vertical direction and a horizontal direction or is performed at a plurality of stages to divide an input signal into different bands of two-dimensional frequency bands shown. In particular, frequencies of an image can be represented on a two-dimensional area as seen in FIG. 8A. The two-dimensional area is divided into a plurality of frequency areas as seen, for example, in FIG. 8B. Signals of each of the divisional frequency areas make subband signals.
Such subband signals are, for example, quantized by linear quantization and then coded by entropy coding (refer to H. Gharavi and A. Tabatabai, "Sub-Band Coding of Monochrome and Color Images", IEEE Trans. on Circuits and Systems, Vol. 35, No. 2, February 1988, pp.207-214).
FIG. 9 shows an exemplary one of conventional image coding apparatus which perform subband division and entropy coding. Referring to FIG. 9, an input image signal is divided by subband division by a subband division section 1 to produce a plurality of subband signals, and each of the subband signals is quantized by a linear quantization section 6 so that a quantization index is outputted from the linear quantization section 6. Such quantization indices are coded by an entropy coding section 7.
In the image coding apparatus shown in FIG. 9, the entropy coding section 7 processes such that a short codeword is applied to a quantization index which appears frequently whereas a long codeword is applied to another quantization index which appears less frequently so that the entire code amount may be minimized. In other words, in order to determine the overall code amount, the frequency distribution of quantization indices must be found out.
However, there is no definite relational expression between the magnitude of the step width in linear quantization and the frequency distribution of quantization indices. Unless the step width is determined, the frequency distribution of quantization indices is not determined, but in order to determine the step width, the frequency distribution of quantization indices is required.
After all, in order to achieve accurate code amount control, recursive processing is required, and accordingly, the processing time cannot be calculated in advance.
In particular, as seen from FIG. 10, a temporary step width is determined temporarily (step 101), and the frequency distribution of quantization indices is calculated in accordance with the step width and then the code amount is estimated from the frequency distribution of quantization indices (step 102). Then, if the thus estimated code amount is excessively great, the step width is increased to decrease the code amount, but on the contrary if the estimated code amount is excessively small, the step width is decreased to increase the code amount. By such feedback operation, a step width with which an aimed code amount is obtained is determined.
However, when a step width is determined by such recursive processing, the processing time until a final step width is determined cannot be calculated in advance, and accordingly, there is a drawback in that the method cannot be applied to an apparatus in which the time required for coding is limited.